airo-linux-gen80211

Could someone shed light on the following:
I used a Aironet PC4800 System and a UDP traffic generator to get some
simple perfomance figures of the Aironet devices.
In my first configuration I had a laptop communicating via a AP
(infrastructure mode) to a PC, which is connected to the AP via
Ethernet.
I send very large UDP packets (1472 Byte -> 1514 Byte Ethernet packets)
as fast as my laptop can generate and send the packets (it is definitely
fast enough for the Aironet devices) at 11 Mbit/s and at a good position
(signal level is 63/64). I get a throughput of around 700-750Kbyte/s
(6MBit/s).
Now I use the adhoc mode (just Laptop to Laptop communication) with the
same measurement setup and conditions. I get a throughput of around
3.3MBit/s.
Has someone a good explaination for that? To my knowledge, the
difference between adhoc and infrastructure mode is, that the AP takes
over some cell/network management things (they still use the same medium
access protocol), but I would not await, that it cuts half of the
bandwidth. What is the problem?
There is another strange thing in infrastructure mode. Then transmitting
UDP packets from the PC to the laptop a can achieve about 700Kbyte/s. In
the other direction (from laptop to the PC) I get 100Kbyte less. Any
ideas here?
Cheers
Jean-Pierre